Many electronic devices and systems have the capability to store and retrieve information in a memory structure. A number of different memory structures are used in such systems. One prominent volatile memory is a DRAM structure that allows for high speed and high capacity data storage. Some examples of non-volatile memory structures include ROM, Flash structures, ferroelectric structures (e.g., FeRAM and FeFET devices) and MRAM structures.
With regard to ferroelectric (FE) structures, these structures can be in the form of a capacitor (e.g., a FeRAM) or a transistor (FeFET), where information can be stored as a certain polarization state of the ferroelectric material within the structure. The ferroelectric material that can be used is hafnium dioxide or zirconium dioxide or a solid solution of both transition metal oxides. In the case of pure hafnium oxide, the remnant polarization can be improved by a certain amount of dopant species which has to be incorporated into the HfO2 layer during the deposition.
The ferroelectric material is intended to partially or fully replace the gate oxide of a transistor or the dielectric of a capacitor. The switching is caused by applying an electrical field via voltage between transistor gate and transistor channel. Specially, for n-channel transistors, ferroelectric switching after application of a sufficiently high positive voltage pulse causes a shift of the threshold voltage to lower or negative threshold voltage values. For p-channel transistors a negative voltage pulse causes a shift of the threshold voltage to more positive threshold voltage values.
FeFET memory enjoys a number of advantages over other Flash storage devices. It generally offers faster read and write access times and lower power consumption during write operation due to the different physical storage mechanism. Further, it is comparatively easy to integrate into High-k metal gate CMOS technology. These advantages, and others, may explain the increasing popularity of FeFET memory for embedded storage as well as for stand-alone applications to be adopted in devices such as memory cards, USB flash drives, mobile phones, digital cameras, mass storage devices, MP3 players and the like.
The areal bit-density of a memory is determined by three parameters: the memory cell size, the memory array efficiency that is the ratio between memory array area and overall chip area including the driving circuitry, and the number of bits that are stored within each of the memory cells. In contrast to single-level cell (SLC) memory, which can store only one bit per cell, multi-level cell (MLC) memory has the ability to store more than one bit of data per cell. In an MLC Flash cell, the data is typically stored in the form of 4 or 8 distinguishable threshold voltage levels, thus yielding two or three bits per cell.